1. Field of the Invention
The present invention relates to a process for producing a steel bar wire rod for cold working. More particularly, the present invention is concerned with a process for producing a soft steel bar wire rod for cold working that can improve the softening level after spheroidization annealing to facilitate the subsequent cold working such as cutting, cold forging and machining in the production of various bolt parts, automobile parts, construction machine parts, bearing parts, etc.
2. Description of the Related Art
The majority of various bolt parts, automobile parts, construction machine parts, bearing parts, etc. has hitherto been produced by subjecting a steel bar wire rod to cold forming such as cutting, cold forging or machining. In cold forming, the rolled material is usually so hard that it is difficult to perform cold working. For this reason, spheroidization annealing is conducted prior to cold forming for the purpose of improving the cold workability. At the present time, however, the softening level is not satisfactory, so that, for example, in the case of cold forging, a further softening of the steel material is desired in the art for the purpose of further improving the tool life.
On the other hand, Japanese Examined Patent Publication (Kokoku) No. 41-19283 discloses a method of preliminarily treating a steel for spheroidization annealing characterized in that a steel bar wire rod is subjected to working of 30% or more at a temperature of from 200.degree. C. to the recrystallization temperature (this temperature is 400.degree. C. in the Example). Although according to this method, the spheroidizing of carbide is accelerated by the spheroidization annealing, and the intervals of carbide particles become so small that it is difficult to attain satisfactory softening. For this reason, at the present time, this technique is not always used in the art.
Accordingly, an object of the present invention is to provide a process for producing a soft steel bar wire rod for cold working that can realize an excellent softening level through conventional spheroidization annealing.